As Microsoft had previously announced multiple times, including at X018 last weekend, keyboard and mouse support for Xbox One is here. It’s part of the November update for Xbox One, which is rolling out now, and which you can download for your console at this minute.
Apart from keyboard and mouse support, which, of course, is the big headlining feature, this update also includes updates for the Cortana Alexa skill, an expansion of the search functionality on the console (which will include results from EA Access and Game Pass, among other things now), as well as the introduction of the Amazon Music app, which is now available to download on Xbox One.
You can check out the full details of the update on Microsoft’s blog post announcing it here. If your console has automatic updates enabled, then it will have downloaded the new update already, and you just have to accept it and install it. If not, you can force an update by going into settings yourself.